User base differences. Mario 3D World would outsell GT6 if both were on the same platform with the same userbase.

This^^ Its funny cuz even though nintendo f***ed up with the wiiu people will compare past games with this one and call it a flop.

Next up DK TF. IT'll sell between 1.5mil-2mil but be a flop (too many) compared to DK's (wii) 6mil. 

Pikmin vs Pikmin 3 is more fair to compare. Even 3DS games vs DS games too! 

I somewhat agree with you, except, GT6 should have sold more, but the franchise is in decline. GT 1 and 2 and 3 were the best selling games for the ps1 and ps2, selling double digit millions. Where as the past 3d mario games have only sold tops 5 million each. Mario is a larger franchise, however GT6 is a larger franchise than 3D mario specifically. And install base really is important to take into account though. 3D world has a great attach rate where as GT6 does not.

The way I kinda see it is like this ...

Super Mario 3D World is like a superstar basketball player who plays for a crappy team (the Wii U, lol). Even though he individually puts up a heroic MVP type performance, he doesn't have much help around him and the team struggles to score without him and predictably gets their butts whupped. 

Gran Turismo 6 is an aging basketball player on a perennially good team (the PS3), but because he's getting older, he can't perform like he used to. The rest of the team around him is also a bit on the old side. You're a little bit surprised at how much he's declined, but on the other hand it's kind of expected. 

What?

Why not? I would understand if you told me not to compare the first week because the game on the older console could be far more frontloaded and thus sell much more. But I'm comparing legs. They are having the same legs so far! The old IP on the old console and considered a flop by many is having the same legs as the new star of the quite new Wii U. This is ironic at least.

I couldn't agree more. That's precisely what I think. But, besides those 2 analysis, we can make other comparisons. I found the one of this thread quite interesting.

That is surprisingly fair! Good analogy.